The plumber had the water turned off at the main house valve to fix a pipe leak. I turned it back on after the pipe fix had dried and the water pressure (both hot & cold) is very low from one of the master bedroom faucets. It was fine before the problem. All the other faucets in the house have no water pressure problems, even the other one in the master bedroom. What could cause the low pressure in this one? What should I check?

Unscrew the airator at the end of the faucet, they have a screen that gets clogged. Usually it will unscrew by hand, sometimes you need a pair of channellocks and a rag to keep from maring the finish.
Good luck.